Ionizing radiation is used in medical beam therapy in order to cure diseases or to delay their progress. It is chiefly gamma radiation, X-radiation and electrons that are used as ionizing, high energy beams.
In order to produce an electron beam either for direct therapeutic use or for the production of an X-radiation, it is customary to make use of particle accelerators. In particle accelerators, charged particles are brought by electric fields to high speeds and thus high kinetic energies, the electric fields resulting in the case of some accelerator types from electromagnetic induction in variable magnetic fields. In this case, the particles require a kinetic energy that corresponds to a multiple of their natural rest energy.
In the case of the particle accelerators, a distinction is made between particle accelerators with cyclic acceleration, such as the betatron or cyclotron, for example, and those with rectilinear acceleration. The latter enable a more compact design and also comprise so-called cascade accelerators (also Cockcroft-Walton accelerators), in the case of which a Greinacher circuit that is multiply connected in series (cascaded) can be used to produce a high DC voltage, and thus a strong electric field, by multiplication and rectification of an AC voltage.
The mode of operation of the Greinacher circuit is based on an arrangement of diodes and capacitors. The negative half wave of an AC voltage source charges a first capacitor via a first diode to the voltage of the AC voltage source. In the case of the positive half wave following thereupon, the voltage of the first capacitor is then added to the voltage of the AC voltage source so that a second capacitor is now charged via a second diode to double the output voltage of the AC voltage source. A voltage multiplier is thus obtained by multiple cascading in the manner of a Greinacher cascade. The respectively first capacitors in this case form a first set of capacitors, connected directly in series, of the cascade, while the respectively second capacitors form a corresponding second set. The diodes form the cross connection between the sets.
Comparatively high particle energies in the region of mega-electron volts can be achieved in such a cascade accelerator. However, in this case there is the risk of electric flashovers (air breakdown voltage: 3 kV/mm), particularly with cascade accelerators set up under normal atmospheric pressure, as a result of which the maximum particle energy is undesirably limited.